1. Field of the Invention
This invention relates to the cleaning of surfaces, and more particularly, to methods and apparatus for dry cleaning of surfaces by use of electrostatic fields.
2. Description of Related Art
Particulate contamination is implicated in a large fraction of yield losses in the manufacturing of semiconductor chips and in yield and reliability losses in high density magnetic storage devices. Highly effective cleaning is recognized as one method to reduce such losses.
In order to remove contaminants from a surface the forces of adhesion of the contaminants must be diminished or overcome or both. Wet cleaning with surface active agents is typically one method for accomplishing both.
However, while wet cleaning can be effective in many instances, there are situations where they are disadvantageous, in that:
1. wet cleaning may leave harmful residues or PA1 2. wetting the material may damage it or PA1 3. the drying times may be prohibitive.
In these instances, dry cleaning is to be preferred.
One common method of dry cleaning, using jets of high velocity gas, is often used, but its effectiveness is known to diminish as particle size decreases, and become ineffective on contaminants of diameters of a few microns or smaller. In fact, it is known in the art that the removal of particles smaller than a few microns is difficult. Thus, there is ever increasing demand for a dry method for removing particle contaminants a few microns and smaller in diameter.
Electric fields have also been used as method of dry cleaning to remove charged particles from surfaces. For example, in U.S. Pat. No. 3,536,528 "Electrostatic Cleaner and Method" issued to De Geest, a method and apparatus for removing particles from the surface of a web is taught by exposing one face of the web to a corona discharge. The corona discharge charges the particles which charging assists in overcoming the adhesion forces of the particles. The web is then passed over an electrode of like (repelling) charge on the opposite side of the web and the particles can then be removed with air currents.
Another cleaning system technique which has been suggested previously makes use of a needle-like electrostatically chargeable probe for detaching particles from the surface. This technique is described in "Electrostatic Cleaning System," by D. C. Shang, IBM Technical Disclosure Bulletin, Vol. 22, No. 7, December 1979, pg. 2696. With the probe initially charged with a given electrostatic polarity, it is scanned over the surface whereby oppositely charged particles are attracted to and temporarily attached to the probe. Particles are subsequently removed from the probe at a remote site.
However, both of the above types of cleaning systems are limited to removing what is considered to be relatively large particles greater than 5 micron because of the dielectric field breakdowns at the field strengths required for the smaller particles. Moreover, as these larger particles are able to effectively be removed by other means, electrostatic cleaning systems have been less preferred methods for implementation.
Theory predicts that contaminant particles on a conductive surface will acquire a charge proportional to the particle area and the electric field. Theory also predicts that the particles will experience a net force in the field proportional to the field times the charge. Thus, the force (F) on the sphere is proportional to the square of the sphere diameter (d) and the square of the electric field (E): EQU F.perspectiveto.k E.sup.2 d.sup.2
where k=3.8.times.10.sup.-6 dyn/V.sup.2 when the field is in volts/cm and the particle diameter is in cm, giving a force in dynes. Good approximations of the net force can be obtained by using the predicted charge, the field at the planar surface, and a correction for image-charge attraction of the sphere to the plane. From this net force equation for particles, it is readily understood that electric fields of greater strength are required to remove the particles of smaller diameter.
The dielectric breakdown of gas depends on geometry and gas density and the gas atomic or molecular characteristics. For air at normal temperature and pressure (NTP) in a gap of the order of 1 cm, field breakdown occurs at 10 kV/cm. With fields higher than this, arcing is known to occur.
While much of the previous work in this area was done in air at NTP and was thus limited to about 10 kV/cm, Myazdrikov and Puznov "Instrument for Determining Adhesion Forces in a Surface Particle System," Zavod LAB (USSR), 35 (10): 1265-1267 (1969), did go to somewhat higher fields (30-50 kV/cm) by using high-pressure gas. Air has its minimum dielectric strength at a fraction of an atmosphere. Thus, the use of high pressures and gases other than air (such as sulfur hexafluoride) are approaches to extending this field strength range.
However, to remove particles a few microns and smaller, electrostatic fields of hundreds of kV/cm to one MV/cm are taught to be required from the above force equations. These high field strengths have heretofore been unable to be obtained.
It is therefore an object of the present invention to provide a method and apparatus for cleaning semiconductor device surfaces of micron and sub-micron contamination by employing an electrostatic particle removing system.